Common Errors in Logic and Rhetoric
©2000 Edward G. Rozycki
edited 7/11/12

Errors In The Structure Of Argument


1) An error in argument structure generates problems. It permits false conclusions from true premises. Also, it hides viable alternatives. Faulty arguments do not connect conclusions with the evidence (the premises) offered for them. If the conclusion happens to be true, it is accidental, so far as the premises are concerned.

2) Many people can recognize that the a-examples below are wrong. But when the same faulty structure contains terms that affect them emotionally, see the b-examples, they tend to miss the structural error.

1. Undistributed Middle:

a. Apples are fruits. Oranges are fruits. So apples are oranges.

b. Sam reads Karl Marx, so he must be a communist because Communists read Karl Marx.

The viable alternatives overlooked in these two examples are --: for a: things that are fruits do not all have to be oranges. For b: non-communists, even anti-communists may read Karl Marx.

2. Unwarranted Converse:

a. All dogs are warm-blooded. Your pet is warm-blooded. So it must be a dog.

Possible missed alternative: your pet is a cat and also warm-blooded, since all cats are.

b. John must be a businessman, since businessmen support immigration and so does John.

Possible missed alternative: John may be a school teacher who supports immigration.

3. Some to All

a. Some animals are meat-eaters. Your pet is an animal. So it must be a meat-eater.

b. Some businessmen support unrestricted immigration. John is a businessman, so he must support unrestricted immigration.

4. Negating the Antecedent

a. All oranges are fruit. Since this is not an orange, it is not a fruit.

b. Since union members oppose unrestricted immigration and John is not a union member, he does not oppose unrestricted immigration.

Rhetorical Errors
5. Ad Hominem

a. "You have to be crazy or perverted to believe that just because all dogs are mammals, my dog is a mammal!

b. John supports unrestricted immigration but he's a jerk!

6. Irrelevant Authority

a. You can be confident that advertising tells the truth because nine out of ten TV celebrities engage in making ads.

b. "Liberalism destroys!" my uncle Rush, the famous statistician, always says.